No abstract

In the recent past, several researchers have successfully modeled the complex fatigue behavior of planar twin-roll cast AZ31B alloy sheets. Complex components are usually hot-bent, whereby the microstructure in the hot-bent areas changes significantly. However, studies on the fatigue behavior of hot-bent magnesium alloys are currently lacking. Therefore, a novel, uniaxial hot-bent specimen was developed and optimized with finite element method simulations. Microstructural analyses with the electron backscatter diffraction method reveal that the hot-bending process changes the texture and increases the Schmid factor for basal slip in rolling and transverse direction of the sheet. In the subsequent quasi-static tension and compression tests, anisotropic and asymmetric yield stresses, lower Young’s moduli compared with the as-received material and macroscopic bands of twinned grains are obtained. Finally, the study proves that the recently proposed concept of highly strained volume can accurately estimate the lifetime, even by combining the as-received and hot-bent material in one fatigue model.

Mechanical fatigue tests of unnotched, notched, and bending twin-roll cast AZ31B magnesium alloy specimens are performed in which strain fields are analyzed with digital image correlation. Clearly, delimited macroscopic bands of twinned grains (BTGs) in which the compressive strain is significantly higher compared to the adjacent regions are observed. Conventional fatigue parameters, e.g., the strain amplitude, exhibited higher values within the BTGs. This findings are confirmed by the fact that for all investigated specimens the initial macroscopic cracks are observed within the BTGs. Consequently, for the presented concept of highly strained volume, fatigue parameters are determined from the highly strained regions with high strain amplitudes. This paper focuses on the application of the effective strain amplitude fatigue parameter decomposed in an elastic and plastic portion, the Smith-Watson-Topper fatigue parameter and energy-based fatigue parameters within the concept of highly strained volume. An extended stress-strain hysteresis model is presented to compute stress-strain hystereses for arbitrary load ratios, required to determine the mentioned fatigue parameters. The application and evaluation of five different fatigue parameters within the concept of highly strained volume demonstrates the accurate description of the fatigue life until failure.

Recent studies have shown the change of microstructure during hot-bending in uniaxial specimens made of AZ31B alloy. They also investigated the influence of the changed microstructure on the quasi-static and cyclic material behavior under uniaxial stress states. However, studies on the fatigue behavior of hot-bent structural components in which a multiaxial inhomogeneous stress state occurs are still lacking. For this purpose, a novel hot-bent V-shaped specimen was developed, of which three different variants, each with a different bending radius, were fabricated and investigated. Microstructural analyses reveal that band-like accumulations of twinned grains are already formed in the compressively stressed area of the specimen during the bending process. Force-controlled low-cycle fatigue tests were performed to investigate the twinning evolution after cyclic loading. Subsequent microstructure analyses show that bands of twinned grains are no longer visible but also that the occurrence of twins is evenly distributed. Due to the specimen shape, the specimens are subjected to a multiaxial stress state. During LCF tests, the strain was measured using 3D digital image correlation and fatigue life was modeled successfully with the application of the concept of highly strained volume.